Improve tooltip text visibility for narrow/small screens

[ODILMichelle]

Images from phone:

Landing page

Main data page

[slowe]

This is an attempt at avoiding screen edges although sometimes it doesn't quite position correctly for unknown reasons. Is this good enough?

[slowe]

A slightly off-topic observation: the text colour in the orange tooltip has changed from white to black because I've updated the algorithm that calculates the most contrast-y text colour to the same one we are now using in OI Lume Charts. It does a better job.

[ODILMichelle]

@slowe - tested it, and it seems to do the trick on the two phones I have access to. Which means I'm happy it's done.

If you're happy it is done/you've done the testing you want, please feel free to close it!

[slowe]

I think I've got the best adjustment of tooltips I can do now.